This paper presents a portable ultrasonic flow measurement system based on a new flow tube with the ultrasonic sensor parallel to the direction of liquid flow. Based on the new flow tube structure of the flow measurement system, two flow calculating algorithms are deduced and error analysis is performed. It is found that one of the algorithms can reduce the error caused by the change in sound velocity, which improves the measurement stability of the system and can also suggest ideas for improvement. This system uses the Qs-k ratio curve to correct the influence of uneven flow velocity distribution of the moving fluid. Within the measurement range, experiment shows that the error of the measured value of the system does not exceed 0.2%.